This invention relates to methods for electrochemical processing of a substrate using copper or other metals. More specifically, the invention pertains to triggering and controlling an initial induction period in which the substrate is immersed in an electrochemical bath prior to actual electrochemical processing.
Electroplating has many applications. One very important developing application is in plating copper onto semiconductor wafers to form conductive copper lines for xe2x80x9cwiringxe2x80x9d individual devices of the integrated circuit. Often this electroplating process serves as a step in the damascene fabrication procedure.
Generally, electroplating requires two principal conditions. First, the wafer or other substrate must be immersed in a plating solution. Second, a potential must be applied between the substrate and a counter electrodes, sufficient to initiate the flow of current between the two electrodes and cause electroplating.
In copper electroplating for damascene processing, the sequence and timing of these operations can have important repercussions in the ultimate quality of the plated copper. In one approach, the wafer to be plated is first immersed in the plating solution and then, after immersion, current is applied. The period between immersion and current application is referred to as xe2x80x9cinduction.xe2x80x9d In another approach, a potential is first applied to one of the electrodes, and then the xe2x80x9chotxe2x80x9d wafer is immersed in the plating solution.
Each of these approaches has its own drawbacks. With the first approach, the induction period can be too long. Induction times of greater than about two seconds can lead to dissolution of a thin conductive copper xe2x80x9cseed layerxe2x80x9d typically applied to the wafer prior to plating. Seed layers are generally about 200 to 1500 angstroms thick and are applied by a physical vapor deposition (PVD) process. They are necessary to impart conductivity to the entire wafer active side and facilitate smooth even electroplating. The electroplating process itself provides a much thicker copper layer (about 1-2 micrometers thick) over the entire wafer active surface. During electroplating, cupric ions migrate through a conductive electroplating solution, deposit on the wafer active surface, and are reduced to form copper metal. To increase the overall conductivity of the electroplating solution, hydrogen ions are frequently added, often from dilute sulfuric acid.
While the nominal thickness of the copper seed layer is about 200 to 1500 angstroms, the actual thickness in certain damascene vias and trenches is generally much thinner, sometimes in the neighborhood of 50 to 100 angstroms. Obviously, plating solutions can aggressively attack the thin, delicate copper seed layer. If the induction period is too long (e.g., 2 to 4 seconds), the seed layer within damascene surface features will dissolve, leading to xe2x80x9cvoidsxe2x80x9d in the electroplated copper layer. Such voids may block electrical paths in an integrated circuit or at least reduce the conductance of the electrical path. As a result, individual integrated circuits, and possibly an entire wafer, may have to be discarded. Thus, uncontrolled induction can greatly reduce the yield of an integrated circuit fabrication process.
Unfortunately, available hardware and software for wafer plating provides inadequate mechanisms for determining when a wafer enters the plating bath. In one contemplated approach, the point at which a wafer is immersed in an electroplating solution is approximated using information from the mechanism that is used to lower the wafer into the electroplating solution. Specifically, an electric motor is used to drive a lead screw to which the semiconductor wafer is attached. The lead screw has a particular mark or index that approximates the level at which the wafer enters the plating solution. Given the variability in plating apparatus and plating solution height, this method is fairly inaccurate. Further, the feedback of this information to a controlling processor in the system typically employs a serial line, e.g., an RS-232 serial connection. There is an associated lag time in generating the signal from the lead screw until the signal is received by the controlling processor. Further, the controlling processor may have various other tasks in its queue ahead of handling such signal. All these effects introduce a variability of at least +/xe2x88x920.5 seconds in detecting when a wafer enters a plating bath. Other methods are even cruder and less accurate. Therefore, there is currently no way to control the induction period to a point where seed layer dissolution is not a problem.
In the xe2x80x9chot entryxe2x80x9d approach, electroplating begins immediately upon immersion of the wafer into the plating bath. So there is no induction period and dissolution of the seed layer is not a significant problem. But this approach produces other problems. The initial plating conditions are critical to obtaining high quality electroplated copper layers. Insufficient wetting and unpredictable convection patterns can lead to uncontrolled growth and generally poor electroplating. This, of course, leads to defects in the resulting integrated circuits and poor yields.
Typically, a wafer must be immersed in a solution for a certain amount of time before it has been fully wetted. Depending upon the characteristics of the plating bath and the wafer""s active surface, adequate wetting may take between about 0.25 seconds and 1 second. If plating occurs before the surface is adequately wetted, there will be certain unwetted regions of the surface where no electroplating occurs, at least initially.
An additional problem arises because the wafer can be rotating during electroplating. It is usually rotating as it is immersed into the solution. The rotation and immersion process often generates bubbles that temporarily adhere to the wafer surface. Hence, the wafer should be immersed in the solution for a short period of time before plating begins. Otherwise, bubbles on the active surface will block electroplating in certain regions.
In view of the above issues, the technology of wafer introduction prior to electroplating requires improved systems and methods for preventing defects due to seed layer dissolution, uncontrolled growth, and other related problems.
The present invention provides methods and apparatus for triggering and controlling an initial induction period in which a substrate is immersed in an electrochemical bath prior to actual electrochemical processing. It accomplishes this by sensing a change in cell potential upon immersion of the substrate or a counter electrode in an electrochemical bath. Appropriate logic then holds the cell potential or current at a fixed value for a defined delay period. After that period ends, the logic allows the cell potential or current to increase to a level where electrochemical processing begins.
One aspect of this invention pertains to methods of controlling the induction period of a wafer in an electroplating solution prior to electroplating. This method may be characterized by the following sequence: (a) applying an entry voltage to the wafer or a counter electrode prior to immersing the wafer in the electroplating solution; (b) immersing the wafer in the electroplating solution while the entry voltage is applied; (c) determining that the entry voltage has passed to a trigger voltage; (d) waiting for a defined delay period after the time of the trigger voltage; and (e) beginning to electroplate the wafer after the delay period. Immersing the wafer in the electroplating solution causes the entry voltage to pass below the trigger voltage.
As mentioned, the invention finds particular use in the context of copper electroplating where the electroplating operation begins with a thin (between about 200 and 1500 angstroms) copper seed layer provided on the wafer""s active surface. In this application, the goal is to prevent the seed layer from being dissolved by acid in the electrolyte.
Typically, the delay period will be between about 0.25 and 2 seconds, and more preferably between about 0.5 and 1.5 seconds. In a specific embodiment, the delay period is about 1 second. Typically, the system will maintain a hold current during the delay period. The size of the hold current may be chosen to prevent dissolution but prevent electroplating. In one example, the hold current magnitude is between about 0.05 amps and 0.25 amps. A typical entry voltage is between about 0.2 and 25 volts between the wafer and the counter electrode. In certain preferred embodiments, the trigger voltage is approximately one-half the value of the entry voltage. In a typical case, the trigger voltage is at least about 0.2 volts.
Another aspect of this invention pertains to apparatus for implementing the method of the invention. Included in the apparatus are components for the following: (a) detecting, applying, and controlling the described voltages and currents, (b) supporting and immersing the substrate into the plating bath, and (c) timing the events of the method.
In certain embodiments the apparatus may be characterized by the following elements: (a) a power supply having a current source and at least one of a voltage and a current sensor; and (b) associated logic for maintaining a hold current between the wafer and counter electrode for a defined delay period after determining that the wafer or counter electrode has been immersed in an electroplating solution. The voltage and/or current sensors sense voltage or current between wafer and the counter electrode in an electroplating apparatus. This allows the logic to control current and voltage in a manner consistent with this invention.
The associated logic may be implemented in any suitable manner. Often it will be implemented in computer hardware and associated software for controlling the operation of the computer. The logic initiates the delay period when it determines that the voltage between the wafer and the counter electrode drops below a predefined trigger level. After the delay period, the associated logic causes the voltage between the wafer and counter electrode to increase to a level at which electroplating occurs.
Typically, the power supply will include both a voltage sensor and a current sensor. In a specific embodiment, it can provide voltage in the range of about 0 to about 30 volts and provide current in the range of about 0 to about 30 amps.
These and other features and advantages of the present invention will be described in more detail below with reference to the associated figures.